Field of the Invention
The present invention relates to an adhesive sheet and more specifically relates to an adhesive sheet that can realize bonding between metals, between a metal and an organic material, and between organic materials, has a good adhesiveness even before curing, and can retain an excellent adhesion strength without undergoing a temperature change after curing.
Description of Related Art
Conventional bonding methods for integration of two adherends include bonding by biting, welding, and bonding with an adhesive agent or a pressure-sensitive adhesive agent that are heavily used in respective suitable fields depending upon applications. In recent years, in applications associated with transportations such as automobiles, a weight reduction in car bodies, hybrid cars, and electric cars have become popular in order to solve problems such as prevention of warming and carbon dioxide (CO2) emission. To this end, aluminum, magnesium, and FRPs such as CFRPs (carbon fiber reinforced plastics) and GFRPs (glass fiber reinforced plastics) have become more and more used.
In conventional welding methods, welding between dissimilar materials, for example, between aluminum and iron is very difficult, and, for FRPs of glass fibers and carbon fibers, welding per se is impossible. Accordingly, a bonding method that can strongly bond these materials (adherends) is demanded. Methods using pressure-sensitive adhesive sheets or bonding sheets are adopted as methods for bonding between materials that cannot be bonded by welding and the like. These pressure-sensitive adhesive sheets and bonding sheets are required to realize bonding between metals, between a metal and an organic material, and between organic materials and to have an adhesion strength high enough to be used in structural use, no deterioration in adhesion strength when the temperature has changed, initial adhesiveness, no need to provide a preheating step and the like, good workability and the like.
A die attachment film having a dicing sheet function comprising a base material film, a pressure-sensitive adhesive layer, a base material film, and a film-shaped adhesive layer provided in that order has been proposed as the bonding sheet in Japanese Patent Application Laid-Open NO. 237483/2006 (patent document 1). Regarding the die attachment film, releasing treatment of the interface on the film-shaped adhesive layer side of the base material film and the use of a mixture of an acrylic ester copolymer with a heat curable resin as the film-shaped adhesive are disclosed. The film-shaped adhesive layer in this film has an initial adhesiveness in a temperature range of 15 to 60° C., but on the other hand, has a low adhesive force because the film-shaped adhesive layer is based on the premise that, after processing, the film-shaped adhesive layer is separated.
A metal foil-clad laminated plate is known from Japanese Patent Application Laid-Open No. 181421/1997 (Patent document 2). The metal foil-clad laminated plate is obtained by providing a release film having a heat-curable resin layer on its one side, superimposing a prepreg, obtained by impregnating a sheet-shaped base material with a heat-curable resin and drying the impregnated sheet-shaped base material, on the heat-curable resin layer side of the release film, subjecting the assembly to heat-pressing molding for integration to prepare a plate-like body, subjecting the plate-like body to desired fabrication, separating the release film, superimposing a metal foil on the heat-curable resin layer side, and subjecting the assembly to heat-pressing molding for integration. In the laminated plate production process, two heat curing steps are provided.
Regarding the adhesive sheets, heat-curable adhesive sheets having initial adhesiveness are also known. These adhesive sheets having initial adhesiveness generally comprise an adhesive layer of a heat curable resin to which a pressure-sensitive resin such as an acrylic resin has been added. The addition of the pressure-sensitive resin such as an acrylic resin to the heat-curable resin results in lowered adhesion strength. To overcome these problems, an adhesive sheet that has initial adhesiveness and can satisfy an adhesion strength of 15 to 20 MPa required in automobile applications has been desired.